What's notable about the open source project announced yesterday, Simple API for cloud computing, are the names that are present, IBM, Microsoft and Rackspace, and the names that are not: Amazon, for one, is not a backer, and let's just stop right there.The Simple API for Cloud Applications is an interface that gives enterprise developers and independent software vendors a target to shoot for if they want an application to work with different cloud environments. It is not literally a cross cloud API, enabling an application to work in any cloud. Such a thing does not exist, yet.

Rather, it supplies an API that will allow an application to call services out of any cloud that supports the Simple API. It would even allow an application running in one cloud to call a service out of another or out of multiple other clouds. Simple API enables that action without requiring any changes to the application itself. To do the same thing without Simple API, an application would have to be re-coded to give it an interface to retrieve storage services from a second cloud as well as from Amazon's S3.

In the long run, what's needed for cloud interoperability is something more than Simple API. What's still needed is a cross cloud runtime format for virtual machines, which would allow an application and operating system to be packaged together into a virtual file format, then shipped off to run in the cloud environment that best for it. Such a thing does not exist, yet.

What we do have on that front is a shared import format, where the application and its operating system can be packaged in a neutral format, Open Virtual Format, that can be recognized by different clouds. Once an OVF file migrates in, however, it will find it was traveling on a one-way street and can't migrate out again unless someone in the host cloud environment agrees to reconvert it into OVF. Which cloud vendor today is willing to convert a workload in its own format into that useable by a competing cloud? Such a move would enable the job to migrate away from its initial host. No cloud .

At the same time, cloud environments exist on the public Internet and Internet prizes openness and interoperability. The leading cloud vendor is Amazon and Amazon would like its Web service interfaces and EC2 cloud format to remain a proprietary advantage as long as possible. That's natural enough, but all cloud vendors should draw a lesson from what's happening to Amazon. The Eucalyptus open source project decided that Amazon's infrastructure was too valuable to be left to the Amazon specialists; it built open source equivalents of EC2's provisioning and start up service, its Simple Storage Service and its Elastic Block Storage.

Simple API will complement what Eucalyptus has already done by adding the ability for an application running in a cloud, or for that matter, in the enterprise data center, to call services out of Amazon.

Amazon is at risk of having its pioneering cloud functions surrounded by open source code, precisely because of the value it has succeeded in demonstrating in the cloud. This tide waits for no man, not even (Amazon's CTO)Werner Vogels. Cloud computing is on the rise and Amazon can encourage its flow or watch the waters swirl around it and continue on their way.